1. Field of the Invention
The present invention relates to a measurement substrate, a substrate table, a lithographic apparatus, a method of calculating an angle of an alignment beam of an alignment system, and an alignment verification system.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Before the exposure of the substrate, it must be correctly aligned to ensure that the functional features are imaged onto the correct position of the substrate. Alignment is the process of positioning the image of a specific point on the mask to a specific point on the substrate which is to be exposed. Typically one or more alignment marks, such as a small pattern, are provided on each of the substrate and the mask. A device may include many layers which are built up by successive exposures with intermediate processing steps. Before each exposure, alignment between the marks on the substrate and the mask is performed to minimize any positional error between the new exposure and the previous ones, which error is termed overlay error.
The marks are commonly on the front side of the substrate, but can also be on the backside of the substrate. Marks on the backside of the substrate are used, for example, when exposure is to take place on the opposite side of the substrate. This occurs particularly in the manufacture of micro electro mechanical systems (MEMS), micro opto-electro mechanical systems (MOEMS), or in Micro Systems Technology (MST).
In front-to-backside alignment (FTBA) applications, both sides of a substrate are used for exposure. In such a case, overlay can be defined with which portion or portions on a back side can be positioned with respect to a front side. This is explained in more detail with reference to FIG. 1. FIG. 1 shows a substrate 10 on which a first layer 11 is processed on a first side of the substrate 10. A second layer 12 is processed on a second side of the substrate 10. After having exposed the first layer 11 on the substrate 10 and turning the substrate upside down, the second layer is intended to be exposed on position 13. However, the real, or actual, position it is exposed on is position 12. The difference between the intended position 13 and the actual position 12 is called overlay. As the intended position 13 has an intended relative distance with respect to a virtual position of the first layer 11 (see virtual layer 14), the overlay depends on the position of the virtual layer 14 and thus layer 11 itself. As layer 11 is on the back side of the substrate, and layer 12 on the front side, the overlay described above is referred to as Front-To-Back-Aligned overlay, or FTBA overlay. FTBA overlay is also referred to as FTBA overlay error.
To actually verify a specified FTBA overlay error in a particular lithographic apparatus, a FTBA overlay error measurement method is required. In commonly assigned, co-pending U.S. Patent Application Publication 2006/0023214 A1, incorporated herein by reference, front to backside alignment (FTBA) verification is performed using a glass verification substrate. The verification substrate includes alignment marks on one side of the verification substrate only, for aligning the verification substrate. Alignment of the verification substrate for exposure of the FTBA overlay patterns is performed by processing both layers, each at an other side of the substrate. The overlay patterns for the verification include a set of verification marks. For the required alignments the same set of alignment marks on the same physical side of the substrate is used. The order, first backside or first front side alignment, may be varied. Thus, one layer is exposed by front side alignment and one layer by back side alignment. The verification substrate is transparent for radiation used by the laser of an alignment system. This means that the back side verification marks which are required to determine the FTBA overlay value can be measured by the alignment system from the front side. However, the non-perpendicularity of the alignment beam (i.e. the laser beam) introduces a shift in the determination of the FTBA overlay.